The efficiency of power transfer between an RF source and an RF load is maximized when the output impedance of the source matches the input impedance of the load. That is, when the source impedance and the load impedance are exactly matched, the load absorbs 100% of the signal transmitted by the source (i.e. the forward signal). When the load impedance varies from the source impedance, however, the load does not absorb the entire forward signal. Rather, the load "reflects" a portion of the forward signal back to the source. The reflected portion of the forward signal is referred to as the reflected signal.
Impedance is defined as having a real component and an imaginary component. The real component is related to electrical resistance, and the imaginary component is related to electrical capacitance and/or electrical inductance. Thus, depending on the existence and size of these components (i.e. resistance, capacitance and/or inductance), source impedance and/or load impedance may consist of some combination of the real component and the imaginary component.
Regardless of its components, the impedance of a source and/or a load may vary over time for variety of reasons. For example, the impedance of a load may drift due to a change in the electrical resistance, capacitance and/or impedance of the components which the load is composed. Such drift may be due to temperature changes, dust, mold, and other environmental forces. Moreover, the impedance of a load may change as a result of the load circuitry becoming damaged, disabled or disconnected due to such environmental factors.
For example, in an RF communication system having an RF output circuit (i.e. source) connected to an antenna (i.e. load), the RF output circuit outputs RF signals (i.e. forward signals) to the antenna for transmission over the air. If, however, the impedance of the RF output circuit is not properly matched to the impedance of the antenna input, then a portion of the forward RF signal will reflect back from the antenna to the RF output circuit (i.e. reflected signal). In fact, the greater the mismatch between the RF output circuit and the antenna input, the greater the value of magnitude of the reflected signal. Thus, the value or magnitude of the reflected signal contains information concerning the extent of the mismatch between the source and the load impedance.
Presently, there are instruments, or systems, available that detect the impedance mismatch between a source and a load (i.e. an RF output circuit and an antenna), and adjust the impedance of either the source or the load to reduce the detected mismatch. Such instruments have been employed to assist in matching source and load impedance to optimize long range transmissions of RE signals between RF transmitters and receivers.
One such instrument is disclosed by Smolka in U.S. Pat. No. 3,919,644, issued Nov. 11, 1975, and incorporated herein by reference. Basically, Smolka discloses an automatic antenna coupler operable to measure the real part of an antenna impedance (i.e. load impedance), and adjust the antenna impedance until it matched a source impedance. To do this, Smolka requires that impedance and phase sensors, and a matching circuit be incorporated into the RF communications system containing the antenna and the source. The impedance and phase sensors detect the impedance mismatch between the source and the load, and the matching circuit is used to vary the impedance of the load to reduce the detected impedance mismatch. As a result, the Smolka device requires the addition of a significant number of components to the RF communications system, thus adding significant cost thereto.
Another instrument for detecting and controlling impedance mismatch between an RE source and an RF load is disclosed by Kuecken in U.S. Pat. No. 3,601,717, issued Aug. 21, 1971, and incorporated herein by reference. Kuecken discloses an automatic impedance matching system including an arrangement for adjusting a matching network coupled between the RF source output and the RF load input to achieve the desired impedance matching between the RF source and the RF load. Significantly, the adjustments to the matching circuit are based on a detected relationship between the forward signal and the reflected signal. That is, Kuecken teaches a device operable to identify a relationship between the forward signal and the reflected signal, and to adjust the matching network based on the identified relationship. Thus, to achieve a source-load impedance match, Kuecken requires that the RF communication system containing the RF source and load incorporate numerous and/or costly components including phase sensors, impedance sensors and a matching network. Thus, the Kuecken device adds significant cost to the RF communications system.
Consequently, even though present day devices can be used to detect a mismatch between an RF source and an RF load, such devices add considerable cost to the overall system. Moreover, such devices are not desirable for some present day applications. For example, in an RF communications system wherein it is desired to only monitor the impedance match between an RF source and an antenna to identify whether the antenna is damaged, disconnected, or disabled, the above described devices are not practical. This is due to the fact that they include expensive components designed to provide more extensive functionality than may be needed in the RF communications system just described.